On 7 February 1863 Army of the Potomac General
Order No. 10 specified a designating flag for headquarters of corps:
blue, swallowtailed, with the number of the corps in red on a white botonee-style cross. Then on 12 May 1863 General Order No. 53
specified a complete set of division and brigade flags. Each corps
was assigned a unique badge (a saltire cross for VI Corps) which appeared on
these flags. The corps headquarters flags, however, were not changed
until 1864, when General Order No. 115 specified that they were to
bear their number on the corps badge.

Some sources show
the VI Corps badge as a straight rather than diagonal cross.
However, Flags of the Army of the United States Carried During
the War of the Rebellion, published in 1887 under the direction
of the Quartermaster General, US Army, depicts the badge as a
diagonal cross and I have so illustrated it here. This source also
depicts the distinguishing flags of the Light Division that was part
of VI Corps during the Chancellorsville campaign. This division,
however, was dissolved in June 1963 and it may never have actually
borne the flags illustrated for it here.